Sensors are at the present time manufactured by micro-machining, using proven semi-conductor processing techniques to sculpt patterns onto thin pieces of silicon in order to create microscopic sensors for sensing pressure, heat, acceleration, flow or sound. Because they are made using processes adaptable to large scale manufacturing, the cost of such sensors is limited by testing and assembly requirements, rather than by manufacturing costs.
Small pressure transducers offered the first large scale market for this technology and, for this reason, development work has tended to be concentrated on such transducers. Pressure transducers have therefore been built using these techniques for over a decade, and have established mass markets in the military, aerospace, transportation and medical fields. Specialist sensors, such as accelerometers, mass flow devices and mechanical vales, have, however, been developed in the laboratory. Recently, in anticipation of the automotive market's need for engine knock sensors, smart suspensions and air bags, prototype runs of accelerometers have been made to validate the techniques and processes involved.
However, although their costs of manufacture are low, prior art micro-machined sensors have significant cost components, which are related to both their calibration and to the interfaces to the computers which they are associated.
Thus, a prior art calibrated micro-machined accelerometer transducer provided with interface electronics may cost, for example, $140.00. Of this amount, the cost of the transducer may well be less than $5.00, and the balance of the cost is made up by the costs of calibration, documentation and marketing expenses.
In fact, a major difficulty in producing a low cost accelerometer at high volume lies in the calibration of the accelerometer, which is required to provide a usable product.
At the present time, when an accelerometer has been manufactured, it is given a functional test. Devices which pass this test must then be mounted on an additional circuit board, containing amplifiers, power regulators and compensation circuitry, and must then be calibrated and "adjusted" to meet specifications. This is a time-consuming manual operation, which very substantially increases the price. If the accelerometer cannot be brought to specification, the circuit board must be reworked.
The result of all this work is a transducer which is similar to a laboratory device, which still cannot be directly interfaced to a computer and which requires still more circuitry and consequential expense and calibration time to become useful.
As will be readily appreciated by those skilled in the art, micro-machined sensors develop output values which vary from true sensed inputs in linearity and offset and due to temperature and other effects.
It is, of course, possible to ascertain the value of an input stimulus to the sensor either by employing a reference sensor device or by employing a known stimulus, such as the gravitational field of the earth. Provided that a means is available to calculate the differences between reference values ascertained in this way and the output values of the sensor and its associated circuitry, it is possible to generate a correction table or function, i.e. a plurality of correction values, which can be utilized to provide a more accurate representation of the true sensed input.
The present invention arises from a realization that the determination of these correction values normally requires more processing power than can be contained in the circuitry associated with the sensor and that the correction values need to be provided to such circuitry in a form in which they can be stored and used to continuously correct the output values of the sensor in use.